The present invention relates to a semiconductor memory device which has a back-up battery and can hold data in a nonvolatile fashion.
Conventionally, a semiconductor memory device which holds data in a nonvolatile fashion using a back-up battery is known. FIG. 1 shows such a conventional semiconductor memory device. This memory device comprises an integrated circuit chip 1 of a semiconductor memory, and a main power source 2 and back-up battery 3 which are coupled to a power source terminal VC of this integrated circuit chip 1 through diodes 4 and 5, respectively. Also, a ground terminal VS of the integrated circuit chip 1 is grounded. A primary battery or a secondary battery is used as the back-up battery 3 and supplies a data holding voltage to the semiconductor memory chip 1 in order to hold the data stored in the semiconductor memory 1 when the main power source 2 fails.
In this semiconductor memory device, if a power supply line 6 and a ground line 7 formed outside the integrated circuit chip 1 are short-circuited, the electric power of the back-up battery 3 will be rapidly lost. On the other hand, when a package in which the integrated circuit chip 1 is enclosed is pulled out from a printed circuit board, this IC chip 1 is disconnected from not only the main power source 2 but also from the back-up battery 3, so that the information stored in this IC chip 1 will be lost. Further, when the main power source 2 fails, the back-up battery 3 supplies an operating voltage to a peripheral circuit as well as to a memory cell array in the memory chip 1; therefore, the electric power consumption of this back-up battery 3 is large, thereby shortening its service life.